Would that grandchild (who is without a doubt a citizen of the US, Congress certainly has that power) be an NBC, assuming Congress passes a law that says that grandchild is born a US citizen.
Like it or not since the Constitution didn’t define natural born we are left with Congress defining and redefining the term as they deemed necessary in the years since. That means they could come up with any cockeyed definition they want, like it or not.
I will answer that question. Yes, this child is a natural born citizen of the US.
He or she may also be a dual national under the laws of Ireland, but that ought to be of no concern to the United States.
Imagine the nonsense scenario where North Korea enacted legislation that extended North Korean citizenship to anyone born in the United States or to a US citizen parent. That would give every American dual citizenship. Would we all become ineligible to become President in such a case? That’s clearly absurd.
And we would not need to revoke North Korean citizenship, either. We would only need ignore it. Ted Cruz did not need to revoke his Canadian dual dual citizenship either. Neither does Donald Trump need to revoke his descent-derived British nationality. Both are US citizens by birth and that is the end of it. Whatever laws foreign jurisdictions may enact are strictly their business, and none of ours.